The present invention relates to protective coatings for surfaces of glass forming equipment and methods of applying the same thereto, and more particularly to protective coatings for, and methods of applying such coatings to, plungers for forming glass articles, to replace chrome coatings customarily utilized for coating such plungers.
Chrome plating has been used on glass forming equipment for more than 40 years. Its primary function has been to provide oxidation and corrosion protection to the forming equipment substrate material. Generally the base material utilized in glass forming equipment may be either 420 stainless steel, H13 tool steel or ductile iron, depending upon the application. Although chromium plating of glass forming equipment has been utilized successfully over many years with different glass compositions, serious problems are encountered when the desired product requires a glass composition containing significant amounts of alkali and/or alkaline components. A problem results which is defined as the build-up of a corrosion product on the surface of the chromium plated forming equipment, which results in unacceptable glass matte surface.
The corrosion product is the result of a reaction involving the alkali and alkaline components of the glass, oxygen, and chromium from the chrome plated forming equipment. The reaction products have been identified as chromate compounds, namely K3Na(CrO4)2, BaK2 (CrO4)2 and K3Ba2Cr3O3. If any one of the three reactants, glass components, the oxygen, or the chrome from the chrome-plated forming equipment were removed from the system, the chromate corrosion problem would cease to exist. In view of the fact that the glass composition is usually dictated by the desired end product, and oxygen is in the atmosphere, it becomes obvious that the most practical solution to the problem is to eliminate the chrome plated forming equipment.
From a pure production standpoint, the need to find a replacement for chrome electroplating of forming equipment, is based upon improved performance of the equipment, however environmental issues also adds impetus to the need for change from the use of chrome plating, since there is a need to reduce the environmental risks associated with chrome electroplating. That is, there is a global trend of hexavalent chrome emission reduction, and the substituted coating materials of the present invention help to reduce such hexavalent chrome emissions.
The chromate corrosion product does not contaminate the glass per se, but accumulates on the chrome-plated glass forming surface and necessitates the replacement thereof in order to produce acceptable ware. In U.S. Pat. No. 5,120,341 to Nozawa et al., the inventors desire to increase the strength of glass containers by decreasing contaminants actually getting into the glass. In order to reduce glass contamination, the patent suggests the use of a gaseous hydrocarbon to coat the mold parts instead of using oil, which creates bubbles and graphite contamination of the glass. In addition, the patent suggests the coating of a plunger with a ceramic or sprayed metal coating which prevents flaking of an oxide layer from the plunger into the glass. Since the ceramic coating can withstand higher temperatures than the metal coating, the patent suggests coating the tip of the plunger with the ceramic coat and the base of the plunger with a self-fluxing metal coating, which cannot withstand the higher temperatures to which the tip of the plunger is exposed.
Further, the sprayed metal coating is less abrasive than the ceramic coating and thus is utilized at the base of the plunger so as to not scratch the glass container about the mouth portion. In another embodiment, the patent suggests the use of a blown air curtain about the plunger which not only cools the plunger but is used mainly in order to prevent foreign substances from attaching to the outside surface of the plunger, which foreign substances would contaminate the newly formed glass. Finally, the patent also suggests the use of an abrasive proof layer between the moving surfaces of the plunger assembly to prevent metal dusts from contaminating the glass, which dust particles would decrease the impact strength of the formed glass container. Unlike the Nozawa et al. Patent, the present invention is not concerned with the contamination of the glass per se, but with the provision of a protective coating to protect substrate material of glass forming equipment, such as a plunger, from oxidation and corrosion.
However, in U.S. Pat. No. 4,830,655 to Franek et al., the inventors were looking for a material to avoid the chemical incompatibilities of known mold materials and glass melts in the forming of glass optical elements with high surface quality. The mold means of the Franek et al. Patent must have a monocrystalline structure and is made from material such as Al2O3, Cr2O3 (which is being avoided in the present invention), MgAl2O4 and/or ZrO2. The shaping device itself is prepared by means of chip forming shaping processes (for example boring, sawing, turning, milling, etc.), from a solid piece of material with subsequent final processing of the mold surfaces (honing, polishing, burnishing, glazing, buffing, etc.). Thus, it can be seen that the Franek et al. Patent does not even contemplate the utilization of a coating applied to a substrate, but actually manufactures the shaping device from a block of material having a monocrystalline structure.
Again, unlike the Franek et al. Patent which forms its shaping device from a solid piece of material by machining the material, the present invention applies a protective coating to additional substrate material for forming glass articles. Thus it has been an object of the invention to not only avoid the utilization of chrome as a plating material, but also to provide an improved corrosion and oxidation resistant coating for glass forming equipment.
The present invention sets forth corrosion and oxidation resistant coatings and methods of applying the same to glass forming equipment to replace the use of chrome as a plating material on such equipment. Coating materials such as Al2O3, and Ni/V may be utilized and an undercoat of Ni or nickel alloy may be employed. The method of deposition onto the substrate of the forming equipment may include sputter coating and electron beam physical vapor deposition.
The criteria for material selection was based primarily upon the assumed compatibility of a material with the hot glass forming environment, specifically high temperature oxidation and corrosion resistance. Other factors such as material toxicity and availability were also considered. Initially 28 materials were selected to be deposited upon test samples in order to have a broad base from which to select the most optimum coating material.
However, since the problem of the build-up of a corrosion product manifests itself primarily on plunger surfaces utilized to form TV panels, it was important to design a test to model the production pressing process and simulate actual conditions for life and performance predictions specific to such problem. Therefore, specifically designated coated samples were cyclicly immersed in TV panel glass under controlled conditions at elevated temperatures. The controlled test conditions were glass temperature, plunger temperature, dwell (immersion) time, cycle time and test duration.
Since the initial test for the 28 selected materials was designed to simulate production process conditions, the glass temperature was maintained at 1000xc2x0 C. with the plunger surface temperature controlled at 600xc2x0 C. Automatic temperature control of the plunger was accomplished by blowing cooling air on the backside of the test plungers. A thermocouple linked to a pneumatically controlled valve regulated the amount of cooling air applied to the plunger. The initial control set was determined with the aid of a contact pyrometer in the plunger up position. The plunger and glass temperatures were collected throughout the test periods. At the end of each test the data was plotted to verify that stable conditions existed throughout the duration of the test.
Dwell and cycle times were set at 12 seconds and 4 cuts/minute, respectively. These times were based on the pressing of a 27 inch TV panel. In the down, or dwell position, the coated plunger was immersed in hot glass contained in a crucible inside an electric box furnace. When the dwell time cycle was reached, the plunger was retracted up and out of the glass through a hole in the furnace lid, stopping approximately 11 inches above the furnace. Test duration was standardized at 48 hours.
The test plungers were made of 420 stainless steel to represent the substrate material commonly used for plungers. A blind hole was machined into the backside of the plunger for insertion of a thermocouple to control plunger temperature. Further, a design incorporating a high temperature gasket and seating flange was used to prevent cooling air from escaping from the furnace chamber, since it was felt that excess oxygen at the coating/glass interface would skew test results. The test plungers were polished and stippled as per the standard production process.
The following is a list of the 28 materials which were deposited upon the test plunger samples and the deposition process utilized.
A target deposition thickness range of about 5-10 xcexcm was targeted, based upon the need for adequate protection from ware and to insure oxidation/corrosion protection of the 420 SS substrate material. It was felt that too thin of a coating may not hold up to the abrasive wear encountered in the production forming process over the course of long runs. Conversely, if the coating was too thick, detrimental residual tensile stresses could build-up in the deposition initiating cracks, which offer a pathway to the unprotected substrate.
After comparing the attributes of the major families of deposition processes, as they relate to the present invention, it was decided that physical vapor deposition (PVD) was best suited to meet present requirements. Chemical vapor deposition (CVD), while offering dense, well adhered depositions, requires process temperatures in excess of the base metals upper limit of 600xc2x0 C. Processing above this temperature would negatively impact the mechanical properties of the 420 SS substrate as well as raise the risk of dimensional distortion.
The state of the art in spray coating technology, in terms of density and microstructure homogeneity has not advanced to the point of being useable for TV panel forming equipment applications. Plasma spray coating does not produce acceptable densities or uniformity. Another shortcoming of the spray coating technology is the need for additional finishing steps once the deposition has been completed. Electroplating is a proven technology in the glass forming industry, however, it is relatively inflexible in terms of coating materials options. The two greatest detriments associated with electroplating, however, are the environmental risks and the cost of compliance. The PVD process, especially electron beam PVD (EB-PVD) appears to satisfy several important criteria, which is further enhanced with ion beam assisted deposition (IBAD). The EB-PVD with IBAD is preferred since it has the capability of producing dense, tightly adhered coatings, with acceptable deposition rates and under controllable conditions without environment risk.
The coatings were examined for adherence, oxide formation, pitting or other indications of corrosion. Surface roughness, which was hoped to have been a key quantitative measure, displayed no correlation with performance. The most common reason for failure was poor oxidation resistance, poor in relative terms meaning that oxidation resistance was not equal to chrome electroplate. The focus of the invention has been to identify and apply materials that display the potential to exceed the performance of chrome electroplate.
Upon reviewing the results of the tests of the 28 materials deposited on the test plungers, it was determined that both Al2O3 and MgO looked exceptional after 48 hours testing. All of the coatings were deposited via the PVD process without a bond coat, with the Al2O3 and MgO being deposited by the electron beam physical vapor deposition process whereas the Ni/V was deposited by a sputtered physical vapor deposition process. Although the differences in coefficient of thermal expansion of MgO and Al2O3 with 420 SS slightly favors MgO, Al2O3 was favored based on the overall appearance of the two coatings, and the lower likelihood of sublimation with Al2O3. Further, it may be desirable to use a bond coat to match expansion coefficients between the Al2O3 and the 420 SS, such as Ni or a nickel alloy such as CoNiCrAlY, which can be applied during the same EB-PVD process that applies the Al2O3.